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diff --git a/Documentation/arm/Booting b/Documentation/arm/Booting new file mode 100644 index 00000000..a341d87d --- /dev/null +++ b/Documentation/arm/Booting @@ -0,0 +1,171 @@ + Booting ARM Linux + ================= + +Author: Russell King +Date : 18 May 2002 + +The following documentation is relevant to 2.4.18-rmk6 and beyond. + +In order to boot ARM Linux, you require a boot loader, which is a small +program that runs before the main kernel. The boot loader is expected +to initialise various devices, and eventually call the Linux kernel, +passing information to the kernel. + +Essentially, the boot loader should provide (as a minimum) the +following: + +1. Setup and initialise the RAM. +2. Initialise one serial port. +3. Detect the machine type. +4. Setup the kernel tagged list. +5. Call the kernel image. + + +1. Setup and initialise RAM +--------------------------- + +Existing boot loaders: MANDATORY +New boot loaders: MANDATORY + +The boot loader is expected to find and initialise all RAM that the +kernel will use for volatile data storage in the system. It performs +this in a machine dependent manner. (It may use internal algorithms +to automatically locate and size all RAM, or it may use knowledge of +the RAM in the machine, or any other method the boot loader designer +sees fit.) + + +2. Initialise one serial port +----------------------------- + +Existing boot loaders: OPTIONAL, RECOMMENDED +New boot loaders: OPTIONAL, RECOMMENDED + +The boot loader should initialise and enable one serial port on the +target. This allows the kernel serial driver to automatically detect +which serial port it should use for the kernel console (generally +used for debugging purposes, or communication with the target.) + +As an alternative, the boot loader can pass the relevant 'console=' +option to the kernel via the tagged lists specifying the port, and +serial format options as described in + + Documentation/kernel-parameters.txt. + + +3. Detect the machine type +-------------------------- + +Existing boot loaders: OPTIONAL +New boot loaders: MANDATORY + +The boot loader should detect the machine type its running on by some +method. Whether this is a hard coded value or some algorithm that +looks at the connected hardware is beyond the scope of this document. +The boot loader must ultimately be able to provide a MACH_TYPE_xxx +value to the kernel. (see linux/arch/arm/tools/mach-types). + +4. Setup boot data +------------------ + +Existing boot loaders: OPTIONAL, HIGHLY RECOMMENDED +New boot loaders: MANDATORY + +The boot loader must provide either a tagged list or a dtb image for +passing configuration data to the kernel. The physical address of the +boot data is passed to the kernel in register r2. + +4a. Setup the kernel tagged list +-------------------------------- + +The boot loader must create and initialise the kernel tagged list. +A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE. +The ATAG_CORE tag may or may not be empty. An empty ATAG_CORE tag +has the size field set to '2' (0x00000002). The ATAG_NONE must set +the size field to zero. + +Any number of tags can be placed in the list. It is undefined +whether a repeated tag appends to the information carried by the +previous tag, or whether it replaces the information in its +entirety; some tags behave as the former, others the latter. + +The boot loader must pass at a minimum the size and location of +the system memory, and root filesystem location. Therefore, the +minimum tagged list should look: + + +-----------+ +base -> | ATAG_CORE | | + +-----------+ | + | ATAG_MEM | | increasing address + +-----------+ | + | ATAG_NONE | | + +-----------+ v + +The tagged list should be stored in system RAM. + +The tagged list must be placed in a region of memory where neither +the kernel decompressor nor initrd 'bootp' program will overwrite +it. The recommended placement is in the first 16KiB of RAM. + +4b. Setup the device tree +------------------------- + +The boot loader must load a device tree image (dtb) into system ram +at a 64bit aligned address and initialize it with the boot data. The +dtb format is documented in Documentation/devicetree/booting-without-of.txt. +The kernel will look for the dtb magic value of 0xd00dfeed at the dtb +physical address to determine if a dtb has been passed instead of a +tagged list. + +The boot loader must pass at a minimum the size and location of the +system memory, and the root filesystem location. The dtb must be +placed in a region of memory where the kernel decompressor will not +overwrite it. The recommended placement is in the first 16KiB of RAM +with the caveat that it may not be located at physical address 0 since +the kernel interprets a value of 0 in r2 to mean neither a tagged list +nor a dtb were passed. + +5. Calling the kernel image +--------------------------- + +Existing boot loaders: MANDATORY +New boot loaders: MANDATORY + +There are two options for calling the kernel zImage. If the zImage +is stored in flash, and is linked correctly to be run from flash, +then it is legal for the boot loader to call the zImage in flash +directly. + +The zImage may also be placed in system RAM (at any location) and +called there. Note that the kernel uses 16K of RAM below the image +to store page tables. The recommended placement is 32KiB into RAM. + +In either case, the following conditions must be met: + +- Quiesce all DMA capable devices so that memory does not get + corrupted by bogus network packets or disk data. This will save + you many hours of debug. + +- CPU register settings + r0 = 0, + r1 = machine type number discovered in (3) above. + r2 = physical address of tagged list in system RAM, or + physical address of device tree block (dtb) in system RAM + +- CPU mode + All forms of interrupts must be disabled (IRQs and FIQs) + The CPU must be in SVC mode. (A special exception exists for Angel) + +- Caches, MMUs + The MMU must be off. + Instruction cache may be on or off. + Data cache must be off. + +- The boot loader is expected to call the kernel image by jumping + directly to the first instruction of the kernel image. + + On CPUs supporting the ARM instruction set, the entry must be + made in ARM state, even for a Thumb-2 kernel. + + On CPUs supporting only the Thumb instruction set such as + Cortex-M class CPUs, the entry must be made in Thumb state. |